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Direct photon physics in heavy ion collisions ~Current Status and Future~

Direct photon physics in heavy ion collisions ~Current Status and Future~. Takao Sakaguchi BNL. Electromagnetic probes (was challenging). Photon Production: Yield   s. Production Process Compton and annihilation (LO, direct) Fragmentation (NLO) Escape the system unscathed

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Direct photon physics in heavy ion collisions ~Current Status and Future~

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  1. Direct photon physics in heavy ion collisions~Current Status and Future~ Takao Sakaguchi BNL T. Sakaguchi, RBRC lunch meeting

  2. Electromagnetic probes (was challenging) Photon Production: Yield  s • Production Process • Compton and annihilation (LO, direct) • Fragmentation (NLO) • Escape the system unscathed • Carry dynamical information of the state • Temperature, Degrees of freedom • Immune from hadronization (fragmentation) process at leading order • Initial state nuclear effect • Cronin effect (kTbroardening) e+ g* e- g T. Sakaguchi, RBRC lunch meeting

  3. Before RHIC In 1986, search for direct photon started in heavy ion collisions at CERN Upper limits published in 1996 from WA80(S+Au at 200GeV/u Followed by WA93 Third generation experiment, WA98, showed the first significant result Pb+PbsNN=17.3GeV, PRL85, 3595(2000). p+Pb data shows initial nuclear effect Baumann, QM2008 T. Sakaguchi, RBRC lunch meeting

  4. Blue line: Ncoll scaled p+p cross-section First gdir in Au+Au (hard scattering) • Au+Au = p+px TAB holds – pQCD factorization works • NLO pQCD works. Non-pert. QCD may work in Au+Au system T. Sakaguchi, RBRC lunch meeting

  5. How direct photons are measured T. Sakaguchi, RBRC lunch meeting

  6. 0  PHENIX Detector • Real Photon measurement • EMCal(PbSc, PbGl): Energy measurement and identification of photons • Tracking(DC, PC): Veto to charged particles • Dilepton measurement • RICH: Identify electrons • EMCal(PbSc, PbGl): Identify electrons • Tracking(DC, PC): Momentum measurement of electrons p0 efficiency Invariant Mass(pT=4GeV, peripheral) T. Sakaguchi, RBRC lunch meeting

  7. Inclusive photon Direct  hadron decay How to extract direct photons • Statistically subtract photon contributions from p0/h/h’/w • Measure or estimate yield of these hadrons • Measure: Reconstruct hadrons via 2g invariant mass in EMCal • Mass = (2E1E2(1-cosq))1/2 • Or, tag photons that are likely from these hadrons event-by-event • Possible if density of produced particles is low (p+p or d+Au) • Subtract remaining background contributions: • Photons that are not from collision vertex • Hadrons that are misidentified as photons • Correct for detection efficiency of photons • Signal is very small. • ~ 5% S/B in 1-3GeV/c • Extremely difficult T. Sakaguchi, RBRC lunch meeting

  8. Double ratio (double g/p0 ratio) • Then, we make this. Taking g/p0 ratio cancels out systematic errors on energy scale measurement T. Sakaguchi, RBRC lunch meeting

  9. Possible sources of photons See e.g., Turbide, Gale, Jeon and Moore, PRC 72, 014906 (2005) Rate hard scatt HadronGas jet Brems. parton-medium interaction jet-thermal sQGP sQGP hadron gas hadron decays Jet-Thermal Jet Brems. log t 1 107 10 (fm/c) Eg Hard Scatt T. Sakaguchi, RBRC lunch meeting

  10. photons dileptons fB: Bose dist. em: photon self energy Difficult objects! Photons from QGP~big challenge~ • Thermal radiation from QGP (1<pT<3GeV) • S/B is ~5-10% • Spectrum is exponential. One can extract temperature, dof, etc.. • Hadron-gas interaction (pT<1GeV/c): ()  (), K*  K Interesting, but S/B is small S/B ratio 5 4 3 2 1 T. Sakaguchi, RBRC lunch meeting

  11. New production mechanism introduced Jet-photon conversion Jet in-medium bremsstrahlung • Bremsstrahlung from hard scattered partons in medium (Jet in-medium bremsstrahlung) • Compton scattering of hard scattered and thermal partons (Jet-photon conversion) Both are “thermal  hard” Turbide et al., PRC72, 014906 (2005) R. Fries et al., PRC72, 041902 (2005) Turbide et al., PRC77, 024909 (2008) Liu et al., arXiv:0712.3619, etc.. T. Sakaguchi, RBRC lunch meeting

  12. A plate ~After cooking up ingredients~ T. Sakaguchi, RBRC lunch meeting

  13. Adding virtuality in photon measurement hard scatt jet Brems. parton-medium interaction jet-thermal sQGP hadron gas hadron decays g*  e+e- virtuality log t 1 107 0.5 10 (fm/c) 1 By selecting masses, hadron decay backgrounds are significantly reduced. (e.g., M>0.135GeV/c2) Mass (GeV/c2) T. Sakaguchi, RBRC lunch meeting

  14. Focus on the mass region where p0 contribution dies out For M<<pT and M<300MeV/c2 qq->* contribution is small Mainly from internal conversion of photons Can be converted to real photon yield using Kroll-Wada formula Known as the formula for Dalitz decay spectra Low pT photons with very small mass e+ Compton e- g* q g q PRL104,132301(2010), arXiv:0804.4168 One parameter fit: (1-r)fc + rfd fc: cocktail calc., fd: direct photon calc. Internal conv. T. Sakaguchi, RBRC lunch meeting

  15. γ e- e+ e+ π0 e+ e- π0 π0 γ e- γ Dilepton Analysis • Reconstruct Mass and pT of e+e- • Same as real photons • Identify conversion photons in beam pipe using and reject them • Subtract combinatorial background • Apply efficiency correction • Subtract additional correlated background: • Back-to-back jet contribution • well understood from MC • Compare with known hadronic sources T. Sakaguchi, RBRC lunch meeting

  16. System size dependence of g fraction • g fraction = Yielddirect / Yieldinclusive • Largest excess above pQCD is seen at Au+Au. • Moderately in Cu+Cu also. Excess also in Cu+Cu No excess in d+Au (no medium) T. Sakaguchi, RBRC lunch meeting

  17. Low pT photons in Au+Au (thermal?) Won Nishina memorial prize! • Inclusive photon × gdir/ginc • Fitted the spectra with p+p fit + exponential function • Tave = 221  19stat  19syst MeV (Minimum Bias) • Nuclear effect measured in d+Au does not explain the photons in Au+Au PRL104,132301(2010), arXiv:0804.4168 Au+Au d+AuMin. Bias T. Sakaguchi, RBRC lunch meeting

  18. Initial temperature at Au+Au • Initial temperature Ti • 300 ~ 600 MeV (different assumptions) • Depends on thermalization time t0 Tc~170MeV from lattice QCD  PHENIX, Phys. Rev. C 81, 034911 (2010)  Theory calculations: d’Enterria, Peressounko, EPJ46, 451 Huovinen, Ruuskanen, Rasanen, PLB535, 109 Srivastava, Sinha, PRC 64, 034902 Turbide, Rapp, Gale, PRC69, 014903 Liu et al., PRC79, 014905 Alam et al., PRC63, 021901(R) T. Sakaguchi, RBRC lunch meeting

  19. Direct photon v2 T. Sakaguchi, RBRC lunch meeting

  20. jet photon conversion Thermal photons Bremsstrahlung (energy loss) v2 > 0 jet v2 < 0 Direct photon v2~a photon source detector~ • Depending the process of photon production, angular distributions of direct photons may vary • Jet-photon conversion, in-medium bremsstrahlung (v2<0) • Turbide, et al., PRL96, 032303(2006), etc.. For prompt photons: v2~0 Turbide et al., PRC77, 024909 (2008) T. Sakaguchi, RBRC lunch meeting

  21. Inclusive photon v2 Calculation of direct photon v2 = inclusive photon v2 - background photon v2(p0,h, etc) • R comes from virtual photon measurement Au+Au@200 GeV minimum bias inclusive photon v2 preliminary T. Sakaguchi, RBRC lunch meeting

  22. Inclusive photon and p0v2 • p0v2 • similar to inclusive photon v2 • Two interpretations • There are no direct photons • Direct photon v2 is similar to inclusive photon v2 Au+Au@200 GeV minimum bias p0v2 inclusive photon v2 preliminary T. Sakaguchi, RBRC lunch meeting

  23. Direct photon v2 • Very large flow in low pT • v2 goes to 0 at high pT • Hard scattered photons dominate Au+Au@200 GeV minimum bias preliminary PHENIX, arXiv:1105.4126 T. Sakaguchi, RBRC lunch meeting

  24. Comparison with models. No success.. • Later thermalization gives larger v2 (QGP photons) • Large photon flow is not explained bymodels for QGP Hydro after t0 Curves: Holopainen, Räsänen, Eskola., arXiv:1104.5371v1 thermal diluted by prompt Chatterjee, SrivastavaPRC79, 021901 (2009) T. Sakaguchi, RBRC lunch meeting

  25. This fits to data well, but.. • Large flow can not be produced in partonic phase, but could be in hadron gas phase • This model changed ingredients of photon spectra drastically! • We realized the importance of the data… thermal + prim. g van Hees, Gale, Rapp, PRC84, 054906 (2011) T. Sakaguchi, RBRC lunch meeting

  26. New degree of freedom? T. Sakaguchi, RBRC lunch meeting

  27. Next step in photons • We might have found that the QGP is formed • High enough temperature to induce phase transition • Need even precise measurement with larger statistics • How does the system thermalize? • In ~0.3fm/c ? How? • A hypothesis says at 0.3fm/c, the system is not thermalized • What happens in the pre-equilibrium state? • Longitudinal expansion. Landau? Bjorken? • What it the initial state condition? Glasma? • Penetrating probe might shed light on the pre-equilibrium states and thermalization mechanism T. Sakaguchi, RBRC lunch meeting

  28. Rapidity as a clock of system evolution • Since the thermalization time is very fast, let’s base on Landau picture (extreme case) • Less thermal photons flying to higher rapidity (g1) may be produced than those to mid-rapidity (g2) • with refer to the QGP formation time. • dz ~ 2R/100, dx ~ 2R • One could see more photons produced in pre-equilibrium states • Rapidity dependence photon measurement may play a role as a system clock g2 dx ~ 2R g1 dz ~ 2R/100 T. Sakaguchi, RBRC lunch meeting

  29. Landau and Bjorkenexpansion models central collision of equal nuclei at differ mostly by initial conditions space-time rapidity proper time T. Sakaguchi, RBRC lunch meeting

  30. T. Renk, PRC71, 064905(2005) Rapidity dependence ~system expansion~ • Forward direct photons shed light on time evolution scenario • Real photons, g*->ee, g*->mm T. Sakaguchi, RBRC lunch meeting

  31. CGC -> Glasma -> QGP, how? • Strong gluon field (Glasma) preceded by CGC + fluctuation • Strong color-electric and magnetic field in a flux tube • extended in z-direction • May play an important role on rapid thermalization • Is there any way to detect Glasma state? • Photons from early stages, i.e., high rapidity? T. Sakaguchi, RBRC lunch meeting

  32. Finding the QCD Critical Point Singular point in phase diagram that separates 1st order phase transition (at small T) from smooth cross-over (at small b) • Quark-number scaling of V2 • saturation of flow vs collision energy • /s minimum from flow at critical point • Critical point may be observed via: • fluctuations in <pT> & multiplicity • K/π, π/p, pbar/p chemical equilibrium • RAA vs s, …. • VTX provides large azimuthal acceptance & identification of beam on beam-pipe backgrounds T. Sakaguchi, RBRC lunch meeting

  33. High Rapidity as a high baryon system • Higher the rapidity goes, higher the baryon density we may be able to reach • BRAHMS plot. Another way to access to the critical point? BRAHMS, PRL90, 102301 (2003) T. Sakaguchi, RBRC lunch meeting

  34. √sNN, T and mb.. • By changing rapidity, we can cover the missing region of √sNN, with high statistics. My eye fit My rough stat calc. NPA772(2006)167 T. Sakaguchi, RBRC lunch meeting

  35. Review ~BRAHMS results • Charged hadron results and some pion/proton ratio results • Might be an idea to extend our measurement to p0/direct photons/dileptons BRAHMS, PLB 684(2010)22. BRAHMS, PRL91, 072305(2003). T. Sakaguchi, RBRC lunch meeting

  36. Drell-Yan as an energy loss probe • Genuine process that involves “quark” • Quark energy loss can be measured • Need a lot of help from model calculations Hot matter created in HIC S. Turbide, C. Gale, D. Srivastava, R. Fries, PRC74, 014903 (2006) T. Sakaguchi, RBRC lunch meeting

  37. How about measurement? ~Detector Plan~ EMCal & (Hcal) • Take Axel’sstrawman’s design (in TPD workshop) • Cover’s rapidity range of y = 3-4 Charge VETO pad chamber ~7m ~7m T. Sakaguchi, RBRC lunch meeting

  38. How about measurement?~A technology choice: MPC-EX~ • Muon Piston Calorimeter extension (MPC-EX) (3.1<|h|<3.8) • Shower max detector in front of existing MPC. Now sits at ~1m from IP • Measure direct photons/p0 in forward rapidity region in p+p, p+A • Study of how high in centrality in A+A we can go is on-going • In the future, placing in a very far position (from Interaction Point) would be an option T. Sakaguchi, RBRC lunch meeting

  39. Summary • Interesting physics are explored by direct photon measurements in HI collisions • Hard photons, Thermal photons, elliptic flow of photons • Rapidity may be a new degree of freedom on photon measurement • I would like to see many predictions on direct photons and dileptons at high rapidity! • I’d be happy to be involved in the theory effort, also. T. Sakaguchi, RBRC lunch meeting

  40. Backup T. Sakaguchi, RBRC lunch meeting

  41. Jet-photon conversion LHC Thermal pQCD ~6GeV? ~15GeV? LHC is a good place for thermal photons/dileptons? • A calculation tells that even in low pT region(pT~2GeV/c), jet-photon conversion significantly contributes to total • What do we expect naively? • Jet-Photon conversions NcollNpart (s1/2)8 f(xT), “8” is xT-scaling power • Thermal Photons Npart (equilibrium duration) f( (s1/2)1/4 ) • Bet: LHC sees huge Jet-photon conversion contribution over thermal? • Together with v2 measurement, the “thermal region” would be a new probe of medium response to partons Turbide et al., PRC77, 024909(2008) T. Sakaguchi, RBRC lunch meeting

  42. Direct photon spectra at d+Au and p+p • Excess in d+Au? • No exponential excess • High-pT direct photon results from PHENIX and STAR • d+Au • Agree with TAB scaled pQCD • consistent with PHENIX and STAR • p+p • Agree with pQCD and PHENIX • Low-pT direct photon • No publication data at STAR STAR, Phys.Rev.C81,064904(2010) T. Sakaguchi, RBRC lunch meeting

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